Process for manufacturing uranium oxide powder

ABSTRACT

A process for manufacturing uranium oxide powder from UF 6  which comprises converting UF 6  to UO 2  F 2  by its reaction with excess alcohol in gas phase and further converting the formed UO 2  F 2  to uranium oxide by combusting hydrocarbon formed in the gas phase reaction and the excessive part of alcohol with oxygen containing gas supplied separately and supplying a regulated amount of steam separately to the combustion reaction zone.

BACKGROUND OF THE INVENTION

A present invention relates to a process for manufacturing uranium oxidepowder from UF₆ using a dry conversion, which is low in fluorinecontaent and high in activity.

As conventional processes which manufacture uranium oxide from UF₆ as astarting material by gas phase reaction, there are known two processes,that is, a process which reacts UF₆ with steam and gydrogen gas at hightemperatures and another process which manufactures uranium oxide fromUF₆ in the presence of the flame ignited by hydrogen gas and oxygen gas.The former is described in Japanese Patent Publication No. 18658 of1961, Japanese Patent Laid Ipen No. 92124 of 1981 (U.S. Pat. No.4397824), and the latter is described in Japanese Patent Publication No.10095 of 1966, Japanese Patent Publication No. 24998 of 1976 (U.S. Pat.No. 3796672 Japanese Patent Publication No. 16976 of 1980. Theseprocesses are called a dry conversion process because of theirmanufacturing uranium oxide by gas phase reaction, while another processwhich comprises hydrolyzing UF₆ to UO₂ F₂ solution, adding ammonia orammonia and CO₂ gases to the UO₂ F₂ solution to form ADU or AUC andmanufacturing uranium oxide from ADU or AUC is called a wet conversionprocess.

The conversion process which reacts UF₆ with steam and hydrogen gas athigh temperature is mainly due to the following reaction machanism.##STR1## But, it is known that these reactions are accompanied by manyside reactions at the same time and that UF₄ is formed partly.Therefore, it is also known that the uranium dioxide powder obtained bythe gas phase reaction is relatively high in fluorine content. Further,these reactors must be heated up to high temperatures because UF₆ reactswith steam and hydrogen gas at the high temperatures.

On the other hand, the process which manufactures uranium oxide powderfrom UF₆ in the presence of the flame ignited by hydrogen gas and oxygengas is mainly due to the following reaction mechanism. ##STR2## In thisreaction, when the ratio of oxygen to hydrogen is more excessive, U₃ O₈rich composotion is formed. This gas phase reaction is required tomaintain the hydrogen flame of the range of 600° to 900° C. Therefore, aconsiderably excessive amount of hydrogen gas, in addition to the gasvolume required for the conversion from UF₆ to UO₂, is required andtemperature of 600°-900° C. can be maintained by combustion of theexcessive hydrogen gas.

The reason why the temperature above 600° C. is required in thisreaction is thought to be due to a slow rate of the reaction of UF₆ withhydrogen and requirement of a considerable amount of activating energy.Further, it is known that the uranium oxide obtained by this gas phasereaction is higher in fluorine content than the uranium oxide obtainedby the conventional wet conversion process.

As the uranium oxide powder obtained by the conventional dry conversionprocess is relatively high in fluorine content, it has a undesirableeffect for manufacturing uranium dioxide pellet.

In the process that UF₆ reacts with steam and hydrogen at hightemperatures, the reactor is required to be heated internally. In theprocess which manufactures uranium oxide powder in the presence of theflame ignited by hydrogen and oxygen, considerably excessive hydrogengas is required for UF₆ to maintain the temperature of the reaction zoneat 600°-900° C.

It is known that UF₆ reacts violently with alcohol to form HF,hydrocarbon and UO₂ F₂ or UF₄. This reaction proceeds much faster thanthe reaction of UF₆ with hydrogen gas.

Further, it is known that this reaction proceeds fast at ordinarytemperatures and is exothermic, while the reaction of UF₆ with hydrogengas proceeds slowly even at 600° C. The present invention is based onthese facts as stated above.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a process formanufacturing uranium oxide powder from UF₆ by gas phase reaction inwhich an uranium oxide powder manufactured is low in fluorine content.

Another object of the present invention is to provide a process formanufacturing uranium oxide powder from UF₆ by gas phase reaction inwhich an uranium oxide powder manufactured is high in activity.

A further object of the present invention is to provide a process formanufacturing uranium oxide powder from UF₆ by gas phase reaction inwhich an uranium oxide powder manufactured is a remarkably suitable rawmaterial for uranium dioxide pellet and nuclear fuel.

According to the present invention, there is fundamentally provided aprocess for manufacturing uranium oxide powder from UF₆ which comprisesconverting UF₆ to UO₂ F₂ by reaction of said UF₆ with excess alcohol ingas phase and further converting said formed UO₂ F₂ to uranium oxide bycombusting hydrocarbon thus formed in said gas phase reaction and saidexcessive part of alcohol with oxygen containing gas suppliedseparately.

In the present invention, the fundamental construction thereof describedabove can be added with a step of introducing a regulated amount ofsteam supplied separately to the combustion reaction zone forcontrolling the temperature thereof.

BRIEF DESCRIPTION OF THE DRAWING

The drawing shows apparatus in side view suitable for carrying out apreferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, as alcohol which reacts with UF₆, methylalcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, buthylalcohol, isobutyl alcohol and higher alcohols can be used. But thehigher the alcohol is, the more the reaction mechanism becomes complex,and the combustibility of hydrocarbon, a reaction product of thereaction of UF₆ with alcohol becomes worse. Therefore, lower alcoholssuch as methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcoholare preferable. As the boiling points of these lower alcohols are in therange of 64.1° C.-97.4° C., it is favorable to evaporate such loweralcohols to react with UF₆ gas.

The reactions of UF₆ with these alcohols are shown by the followingreaction formulas (4)-(6), but it is known that in these reactions, HF₄is formed partly. ##STR3##

For reacting UF₆ with alcohol in gas phase, it is preferable to use abinary fluid nozzle. In this case, it is required to make a gas linearvelocity of UF₆ gas to be comparatively large at the end of the nozzleto prevent blockade from forming due to UO₂ F₂, a reaction product.

The amount of alcohol required for reaction with UF₆ is at leastdemanded 2 fold of equivalent, as also shown by the chemical formulas(4)-(6) described above, in the gaseous state heated up to the sametemperature as that of UF₆. But for proceeding completely the reactionwith UF₆, alcohol of 1.05-1.25 fold of reaction equivalent is requiredfor UF₆. In this case, when the excessive amount of alcohol is too morethan the reaction equivalent, the temperature of the flame in combustionbecomes too higher, consequently the activity of the obtained uraniumoxide powder is decreased, and the amount of alcohol which combustswastefully becomes larger uneconomically.

The UF₆ gas and the alcohol gas are blown out through the binary fluidnozzle and a spindle-shaped reaction zone of these gases is formed infront of the head of the nozzle. Excess air or oxygen gas is supplied tothe latter half part of the spindle-shaped reaction zone to ignite anignition device to make a flame-shaped second reaction zone formed.

In the second reaction zone, the excessive amount of the alcohol and thehydrocarbon as ethylen formed in the first reaction zone are combustedand this heat of combustion converts UO₂ F₂ powder and a very smallamount of UF₄ powder formed in the first reaction zone to uranium oxidepowder.

In the present invention, the flame-shaped second reaction zone isformed by combusting the hydrocarbon as ethylene and the excessiveamount of the alcohol, while in the conventional process whichmanufactures uranium oxide powder in the existence of the flame, theflame is formed by combusting an excess of hydrogen gas. But, in thisconventional process, the heat of combustion of hydrogen is 2,580Kcal/m³ and according to comparison of this value with 14,116 Kcal/m³ ofthat of ethylene, 7,749 Kcal/m³ of that of methanol, 14,570 Kcal/m³ ofthat of ethanol and 21,964 Kcal/m³ of that of propanol, the heat ofcombustion of hydrogen is 1/3-1/8 of these heats of combustion,therefore a considerably excessive amount of hydrogen is required forUF₆ to keep the temperature of the flame at 600° C.-900° C.

In the present invention, 1.05-1.25 fold of the reaction equivalent ofalcohol is sufficient for UF₆. Even in this case, the temperature of theflame zone is kept to be in the range of 800° C.-1000° C.

Then, in the present invention, the steam of 110° C.-150° C. is suppliedto the flame zone to control the temperature of the flame zone at 600°C.-800° C. and to make activity of the uranium oxide formed suitable formanufacturing of uranium dioxide pellet.

When the temperature of the flame zone is high, the activity of theuranium oxide powder is decreased due to sintering thereof.

In the present invention thus, by supplying steam for controlling thetemperature of the flame zone, the fluorine contained in the uraniumoxide can be removed as HF, and by oxidizing in the atmospherecontaining the evaporated alcohol, defluorination can be promoted.Therefore, the obtained uranium oxide is considerably lower in fluorinecontent than that of the conventional dry convertion process.

As in the present invention, the uranium oxide powder obtained in thisway is U₃ O₈ powder, it is required to be reduced by hydrogen in arotary kiln or a fluidized bed known to those skilled in the arts forobtaining uraium dioxide powder suitable for nuclear fuel.

The present invention, as described above, provides a process which canmanufacture economically uranium oxide powder containing an extremelylow amount of fluorine in keeping the activity of the powder, and isuseful for manufacturing nuclear fuel.

The drawing shows one example of the reaction apparatus which is used inof the present invention. The apparatus comprises reactor 5, powderreceiving hopper 9 which is connected to the reactor 5 by rotary bulb 8,screw feeder 11 which is driven by motor 10 connected to the lower endof hopper 9, and further receiver 13 with rotary bulb 12 connected toscrew feeder.

Reactor 5 has combustion cylinder 5A which is installed to the reactordiagonally and downward at the lower half part of the reactor and hassintered metal filters 6 at the top. Gas reaction products pass throughfilters 6 and also through path 7 to be introduced to waste gas treatingapparatus 19.

Combustion cylinder 5A is provided with a binary fluid nozzle forintroducing reactants, ignition device 3 (spark device), nozzle 16 forintroducing steam 14, and nozzle 15 for introducing oxygen 20. Theapparatus is preferably made of Ni base superalloy like Hastelloy. Inthe drawing, 20 is oxygen gas, 17 is alcohol gas, 18 if UF₆ gas and N₂gas and 2 is the first reaction zone.

The present invention will be understood more readily with reference tothe following examples. The examples, however, are intended toillustrate the present invention and are not to be construed to limitthe scope of the present invention.

EXAMPLE 1

Reactor 5 shown in the drawing is used. From the inside pipe of a binaryfluid nozzle 1 is blown out nitrogen gas and from the outside pipethereof methyl alcohol is blown out and at the same time from nozzle 15is supplied oxygen gas to ignite an ignition device. When thetemperature of reactor 5 reaches about 200° C., UF₆ gas of which theflow amount is 123 g UF₆ /min is blown out, instead of N₂ gas at theflow rate of 70 m/sec through the inside pipe of the binary fluidnozzle, and the same time the flow amount of methyl alcohol isdetermined to be 2.5 fold of the flow amount of the UF₆ gas. This meansthat methyl alcohol corresponding to 1.25 fold of reaction equivalent ofUF₆ gas is supplied. Consequently, the temperature of the secondreaction zone 4 of the flame zone reaches 900° C., and steam of 120° C.is supplied to control the temperature of the second reaction zone to700° C.

In this way, the reaction proceeds for 15 min., and the U₃ O₈ of 1,450 gis obtained.

Next, the U₃ O₈ is reduced in the hydrogen atmosphere at 630° C. to UO₂powder in a small batch furnace.

The UO₂ powder obtained in this way has a mean particle size (Fsss) of0.65 μm and a specific surface area (BET) of 3.052 m² /g. Further, thefluorine content thereof is 15 ppm.

EXAMPLE 2

The reactor shown the drawing is used. N₂ gas and ethyl alcohol areblown out from the binary fluid nozzle 1 and at the same time O₂ gas isalso supplied to ignite an ignition device 3. When the temperature ofthe reactor 5 is heated up to about 200° C., UF₆ gas of the flow amountof 123 g UF₆ /min is blown out at the flow rate of 80 m/sec from theinner pipe of the binary fluid nozzle instead of N₂ gas and at the sametime the flow amount of the ethyl alcohol is made to be 2.1 fold of thatof the UF₆. This means that ethyl alcohol of 1.05 fold of reactionequivalent for UF₆ is supplied. As the result, the temperature of thesecond reaction zone 4 of the flame zone becomes 1000° C., thereforesteam of 120° C. is supplied to control the temperature of the secondreaction zone 4 to 800° C.

After the reaction proceeds for 17 min., there is obtained U₃ O₈ of1,640 g.

Next the U₃ O₈ is reduced in hydrogen atmosphere at 650° C. using asmall batch furnace to UO₂ powder. The UO₂ powder obtained in this wayhas a specific surface area (BET) of 2.65 m² /g and a mean particle size(Fsss) of 0.68 μm. Further, the fluorine content thereof is 8 ppm.

EXAMPLE 3

The reactor shown in the drawing is used. N₂ gas and propyl alcohol isblown out from the binary fluid nozzle 1, and at the same time oxygengas is also supplied to ignite an ignition device.

When the temperature of the reactor 5 becomes about 200° C., instead ofN₂ gas, UF₆ gas of a flow amount of 123 g UF₆ /min. is blown out at theflow rate of 80 m/sec. At the same time the flow amount of propylalcohol is determined to be 2.1 fold of the flow amount of UF₆ gas. Thismeans that the 1.05 fold of reaction equivalent of propyl alcohol isapplied to UF₆.

In the result, the temperature of the second reaction zone 4 of theflame zone becomes 1,200° C., so the temperature of the second reactionzone 4 is kept to be 800° C. by supplying steam of 120° C. Thus, afterthe reaction of 20 min., there is obtained U₃ O₈ of 1,905 g.

Next, the U₃ O₈ is reduced in hydrogen atmosphere at 650° C. to UO₂powder, by using a small batch furnace. The UO₂ powder obtained in thisway has a mean particle size (Fsss) of 0.70 μm and a specific surfacearea (BET) of 2.51 m² /g.

Further, the fluorine content thereof is 5 ppm.

What is claimed is:
 1. A process for manufacturing uranium oxide powderfrom UF₆ which comprises converting UF₆ to UO₂ F₂ by its reaction withexcess alcohol in gas phase and further converting said formed UO₂ F₂ touranium oxide by combusting hydrocarbon formed in said gas phasereaction and said excessive part of alcohol with oxygen containing gassupplied separately.
 2. A process for manufacturing uranium oxide powderfrom UF₆ which comprises converting UF₆ to UO₂ F₂ by its reaction withexcess alcohol in gas phase and further converting said formed UO₂ F₂ touranium oxide by combusting hydrocarbon formed in said gas phasereaction and said excessive part of alcohol with oxygen containing gassupplied separately and supplying a regulated amount of steam separatelyto said combustion reaction zone.
 3. A process for manufacturing uraniumoxide powder from UF₆ as claimed in claim 1 or 2, wherein said alcoholis a lower alcohol having the number of carbon atom up to
 3. 4. Aprocess for manufacturing uranium oxide powder from UF₆ as claimed inclaim 1 or 2, wherein the amount of said alcohol used is in the range of1.05-1.25 fold of reaction equivalent.
 5. A process for manufacturinguranium oxide powder from UF₆ as claimed in claim 1 or 2, wherein saidUF₆ and said alcohol are blown out through a binary fluid nozzle.
 6. Aprocess for manufacturing uranium oxide from UF₆ as claimed in claim 2,wherein the temperature of said combustion reaction zone is regulated tothe range of 600° to 800° C. by introduction of said steam.